An electronic device, such as a semiconductor laser, is typically mounted on a submount for the purposes of mechanical stability, heat dissipation, and electrical connections to one or more electrical sources (d-c and a-c control voltages or currents, or both). Because of its desirably higher thermal conductivity, diamond appears as an attractive alternative to silicon or to thermally conductive ceramics for the choice of material for the submount.
In general, the submount is in the form of a die that has been diced (cut) from a plate (wafer), and the device is located on a portion of a top surface of the die. Typically, there is also located on the top major surface of the die, among other things, a metallic wire-bonding pad to which one end of a metallic wire is attached, the other end of the wire being attached to an electrode located on a top major surface of the device. In this way, by connecting (attaching) one end of an additional wire to the bonding pad and the other end to an external electrical power source, a low electrical resistance connection is made to the electrical source. This additional wire, however, entails an undesirable additional amount of bonding pad area, as well as introduces an amount of added inductance that is especially undesirable as the a-c frequency of operation of the device increases.
In prior art, when using silicon as the material for the submount, the metallic bonding layer is deposited on the top surface of a silicon dioxide layer that has been formed on the top surface of the silicon wafer (prior to dicing). The purpose of the silicon dioxide layer is to supply electrical isolation of the metallic bonding pad from the (semiconductive) silicon die. Then, for each die to be cut from the wafer, an aperture (via hole) penetrating from the bottom to the top surface of the wafer is wet-etched; and the resulting (sloping) sidewall of the via hole, together with the bottom surface of the wafer, is coated with a metallic layer, such as by evaporation or sputtering. Ordinarily, because silicon is somewhat fragile, only then is the thickness of the silicon wafer reduced to its ultimately desired amount, by lapping, grinding, and polishing the bottom surface of the wafer, whereby the metallic layer on its bottom is also removed; and, after the silicon wafer is thus thinned to the desired thickness, its thus exposed bottom surface is again coated with a metallic layer. The metallic layer located on the bottom surface of the wafer (the "metallized backplane") supplies a desirable means for the added access from the external power source to the bonding pad (through the via hole): after dicing the silicon wafer, electrical contact from the electrical power source directly to the metallized backplane, and hence indirectly to the bonding pad (through the metallic coated sidewall) can easily be made--namely, by soldering the metallized backplane of the submount to the top surface of a metallic mounting body ("platform")--without the need for the undesirable additional wire connection from the power source to the bonding pad located on the top surface of the wafer. Thus, a micro-strip transmission line is formed, the metallized bottom surface of the plate serving as the backplane of the transmission line, having a characteristic impedance that is determined by the various geometric parameters of the resulting configuration and the dielectric constant of the silicon.
As mentioned above, however, the material for the submount, diamond has more attractive qualifies than silicon. In addition, diamond is less fragile than silicon; therefore, a diamond plate does not require thinning after the formation of the via hole. Moreover, by reason of the method used in prior art, the bonding pad extends across the entire top of the aperture in the silicon wafer, whereby the resulting self-supporting portion of the bonding pad is fragile and unreliable during subsequent processing, especially during subsequent bonding of the submount to the platform at necessarily elevated temperatures. More specifically, in prior art, since the via hole is wet-etched, it is wet-etched at a time when the bonding pad has already been formed: if the via hole were to be wet-etched prior to the time when the bonding pad is formed, followed by the formation of the bonding pad itself and a metallic coating on the sidewall of the aperture, then the required electrical connection between the metallic coating on the sidewall and either the bonding pad or the metallized backplane (or both) would be unreliable because of the sharp angular geometry at the respective intersections between the sidewall and either the top surface or the bottom surface of the wafer (or both the top surface and the bottom surface of the wafer).
Although when using a diamond plate as the material for the submount, it is possible to connect a deposited (typically evaporated or sputtered) metallic layer located on the bottom surface of the diamond plate with a deposited metallic layer located on the top surface of the plate by means of a deposited metallic layer located on an end (side) surface of the plate, the deposited metallic layer running from the bottom surface to the top surface of the plate, the resulting connection is weak, and hence unreliable, in regions overlying the top or bottom edges (or both edges) of the plate--i.e., at the intersections of the top and bottom surfaces with the side surface of the plate--again because of the relatively sharp angular geometry at these intersections.
Therefore, it would be desirable to have a method of forming a metallized path connecting opposed metallized surfaces of a diamond plate--or any metallized arbitrary path located on a diamond surface--that mitigates the problems of prior art.